The coating of graphitic or ceramic particles in turbulent layers into which a thermally decomposable gas is introduced is well known. For example, the pyrolytic carbon coating of fuel or breeder kernel materials for nuclear reactors by the introduction of a thermally decomposable gaseous hydrocarbon into a turbulent layer is known, as disclosed in German Pat. No. 1, 808,550. As well as layers of pyrolytic carbon, it is feasible by this method to produce also coatings consisting of other high temperature stable ceramics, or metals, as for example layers of silicon carbide, molybdenum or tungsten. The particular coating material that is deposited and also its structure depend essentially on the nature of the coating gas, on the concentration of the gas components in the gas stream which are decomposed at the decomposition temperature, upon the property of the surface of the articles to be coated, on their temperature and on the total surface of the articles to be coated in the fluidized loose material layer. The commercial value of the process depends primarily upon the formation velocity of the layers and their quality, which must satisfy the requirements of the particular case.
For the coating of larger articles with dimensions of the order of 1 to 30 millimeters with pyrolytic carbon, a process is known that consists in fluidizing the articles to be coated in common with fluidizing particles of a diameter of a few hundred microns (.mu.m) during exposure to a hot coating gas, as referred to in German published patent application (OS) No. 2 311 791. For the formation of relatively thick layers, it is recommended in this process to maintain constant the aggregate surface of the fluidizing particles in the fluidized bed per unit of time and to supply the fluidizing particles and remove them continuously. The fluidizing particles have an assisting role in this process. Cheap fluidizing particles are used that have a higher density than the articles to be coated. The gas velocities necessary for fluidization are then so high that the articles do not sink, nor do the valves get clogged. It is known from German published patent application (OS) No. 2 311 791 to utilize zirconium oxide particles as fluidizing particles for the coating of graphitic hollow spheres. It is not possible in this case, however, to prevent contamination of the pyrolytic carbon that is deposited, at least in the parts per million range, by zirconium oxide.
If it is desired to produce layers of a high purity and free of any particular contamination, as is especially required for atom absorption spectroscopy, fluidizing particles are preferably employed that are of a material corresponding to the material of the layer to be deposited. Thus, for example, in the production of pyrocarbon layers, fluidizing particles of carbon are utilized. In this case, however, the articles to be coated generally have a higher ratio of weight to surface in comparison with the fluidizing particles, so that the driving force in the turbulent layer produced by the gas stream that is introduced is not sufficient to prevent a separation of the articles to be coated from the fluidizing particles. The articles to be coated sink, while the fluidizing particles, as for example graphite pellets, form the actual fluidized bed somewhat above those articles. It is observed that the undesired separation occurs particularly with an increased loading of the turbulent layer with a larger number of articles to be coated, essentially independent of the flow-through rate of the gas.
It is an object of the present invention to overcome these disadvantages and to devise a coating process in which a larger number of particles to be coated per unit volume of fluidized bed can be coated with a layer of higher quality. It is a further object of the invention to provide such a process in an economic and simple form.